Basically, a container is sealed air-tightly after food or drink is filled therein. If a pinhole or a seaming defect exists in the container, or if the content is deteriorated, a vacuum down and a pressure drop may be caused by an intrusion of air or generation of gas, or by a leakage of an infill gas. In order to avoid such disadvantage, the internal pressure of sealed container filled with the contents has been inspected to pick up defective products from a manufacturing line. To this end, as disclosed in Japanese Patent Laid-Opens Nos. 63-67845, 06-213748, and 2002-14813, an inspection of abnormality of the pressure in the sealed container has been carried out by tapping the sealed container and analyzing frequency of the resultant sound or vibrations.
In recent days, canned products containing solid materials or high viscosity content such as corn soup have been available in the market. In the canned product of this kind, the contents or solid materials thereof are likely to adhere to an inner surface of (a sailing or bottom lid of) the container to be tapped unevenly and irregularly. That is, frequency or distribution of the sound or vibration caused by such a tapping inspection may be varied depending on the condition of the can lid. Consequently, the canned product the internal pressure thereof is normal may be judged as a defective product erroneously.
In order to avoid such a disadvantage, an inspecting method or apparatus for inspecting a deformation of the container caused by the internal pressure has been proposed. For example, Japanese Patent Laid-Open No. 8-219915 discloses an equipment for inspecting an internal pressure of a can. The equipment taught by Japanese Patent Laid-Open No. 8-219915 is comprised of an eddy current type distance sensors situated above conveyer for conveying the cans packed in a box. The distance sensor is adapted to measure one point in the center of the cover and two points on the seeming part of the can in the box being conveyed on a conveyor. According to the teachings of Japanese Patent Laid-Open No. 8-219915, the deformation of the center of the cover is calculated based on the measured value, and the internal pressure of the can is inspected by comparing the deformation with a reference value. Meanwhile, Japanese Patent Laid-Open No. 2009-210451 discloses an apparatus for inspecting a can carried in a carton case on a conveyer. The apparatus is adapted to measure a top depth from the upper end of the can to the open tab, and a bottom depth from the lower end of the can to the bottom panel. A total value of those depths is compared with a reference value to determine the internal pressure of the can.